Technically, this isn't a security alert, but Microsoft has released the first post-SP3 rollup fix for Exchange 2000. KB article 813840 links to the list of fixes.

There's a companion set of fixes for the Active Directory Connector. KB article 815452 contains its list of fixes.

UPDATE: Microsoft has pulled the downloadable update, citing mismatches between the rollup binaries and the associated symbol files. They haven't yet provided an ETA for restoring the download, although the KB articles are still there.